The manufacture of electronic integrated circuits relies heavily on the use of image projection techniques to expose resist-coated wafers with light or X-rays. The patterns formed by this exposure determine the various circuit connections and configurations. In any exposure method, accuracy of the projected image is a prime consideration. This accuracy is particularly important in the manufacture of high density random access memories (RAM) in which the yield and ultimately the cost of the components depend heavily on meeting tight exposure placement requirements. With the increasing demand for high performance integrated circuits, the techniques to fabricate semiconductor substrates for microelectronic devices and other purposes have been undergoing continuous development and now include the use of scanning-electron beam lithography systems, both for producing high quality lithographic masks and for direct pattern generation.
Electron beam lithography systems use electron sources that emit electrons at all angles. The electrons are then constrained by the remainder of the system into a narrowly diverging beam. Succeeding lenses then focus the beam into one or more cross-overs before the beam reaches the target. In these systems, electron beams are formed by an electron beam column that, at a minimum, includes an electron source at an object plane and a target at the image plane. Usually the electron beam column includes at least an electron source at the object plane, one or more lenses, one or more apertures, and the target at the image plane. Columns for electron beam lithographic mask exposure include at least an electron source at the object plane, one or more lenses, one or more apertures, one or more deflectors, a set of beam blankers (which can be driven to stop the beam reaching the target), and a target at the image or mask plane.
In direct pattern generation where the electron beam system creates a pattern directly on a chip covered with resist material, the often complicated and time consuming mask-making process is eliminated. However, one of the key economic considerations in a direct electron beam lithography system for a production environment is the throughput achieved by direct writing relative to a system using a series of masks. This is of particular importance, because direct writing is necessarily a serial output process. Hence, time constraints become even more critical in direct pattern generation.
As manufacturers seek ever higher writing speeds, other significant problems also appear. These problems arise often as a result of the relationship among these various parameters. For example, as the writing speed increases, the current density must be increased to maintain the same exposure on the resist. However, higher current densities lead to beam broadening due to electron-electron interactions, thereby deleteriously increasing the line width. Also, a shortened exposure time further requires a shortened blanking time, since the rise time of the blanker is closely related to the accuracy of the exposure of each pixel, and is also a major concern in avoiding extraneous exposure during blanking. Hence, blanking time in raster scan type electron beam devices remains one of the key factors limiting throughput.
The electron optical column delivers a variable sized spot with constant current density, the spot current increasing as the square of spot size. Correct resist exposure requires a certain number of coulombs per unit area. From the resist sensitivity and beam current density we can obtain the time required to expose the area covered by the spot. The time taken to expose a mask is this time multiplied by the mask area divided by the spot area.
For the purpose of understanding the description of the invention, assume the spot to be square, equal to the address grid, and all the mask area to be rastered. For a beam current density J and resist sensitivity S the exposure time per spot (pixel rate) is S/J secs. The given variable is resist sensitivity, beam current density and pixel exposure rate must be matched to achieve the correct dosage.
Over much of the operating range the MEBES machine is not making best use of the available beam current.